1. Field of the Invention
The present invention relates to a melt-stable lactide polymer composition and a process for manufacturing such composition from lactide in the field of degradable polymers.
2. Description of the Prior Art
The continued depletion of landfill space and the problems associated with incineration of waste have led to the need for development of truly biodegradable polymers to be utilized as substitutes for non-biodegradable or partially biodegradable petrochemical-based polymers. The use of lactic acid and lactide to manufacture a biodegradable polymer is well-known in the medical industry. As disclosed by Nieuwenhuis et al. (U.S. Pat. No. 5,053,485), such polymers have been used for making biodegradable sutures, clamps, bone plates and biologically active controlled release devices. Processes developed for the manufacture of polymers to be utilized in the medical industry have incorporated techniques which respond to the need for high purity and biocompatability in the final product. These processes were designed to produce small volumes of high dollar-value products, with less emphasis on manufacturing cost and yield.
In order to meet projected needs for biodegradable packaging materials, others have endeavored to optimize lactide polymer processing systems. Gruber et al. (U.S. Pat. No. 5,142,023) disclose a continuous process for the manufacture of lactide polymers with controlled optical purity from lactic acid having physical properties suitable for replacing present petrochemical-based polymers for packaging, paper-coating and other non-medical industry applications.
Generally, manufacturers of polymers utilizing processes such as those disclosed by Gruber et al. will convert raw material monomers into polymer beads, resins or other pelletized or powdered products. The polymer in this form is then sold to end users who extrude, blow-mold, cast films, blow films, thermoform, injection-mold or fiber-spin the polymer at elevated temperatures to form useful articles. The above processes are collectively referred to as melt-processing. Polymers produced by processes such as those disclosed by Gruber et al., which are to be sold commercially as beads, resins, powders or other non-finished solid forms are generally referred to collectively as polymer resins.
Prior to the present invention, it is believed that there has been no disclosure of a combination of composition control requirements which will lead to the production of commercially viable lactide polymer resins for the melt-processors.
It is generally known that lactide polymers or polylactide are unstable. The concept of instability though, has both negative and positive aspects. The positive aspect is the biodegradation or other forms of degradation to form non-hazardous products when lactide polymers or articles manufactured from lactide polymers are discarded or composted after completing their useful life. The negative aspect of such instability is the degradation of lactide polymers during processing at elevated temperatures as, for example, during melt-processing by end-user purchasers of polymer resins. Thus, the same properties that make lactide polymers desirable as replacements for non-degradable petrochemical polymers also creates undesirable effects during processing which must be overcome.
Lactide polymer degradation at elevated temperature has been the subject of several studies, including: I. C. McNeill and H. A. Leiper, Polymer Degradation and Stability, vol. 11, pp. 267-285 (1985); I. C. McNeill and H. A. Leiper, Polymer Degradation and Stability, vol. 11, pp. 309-326 (1985); M. C. Gupta and V. G. Deshmukh, Colloid & Polymer Science, vol. 260, pp. 308-311 (1982); M. C. Gupta and V. G. Deshmukh, Colloid & Polymer Science, vol. 260, pp. 514-517 (1982); Ingo Luderwald, Dev. Polymer Degradation, vol. 2, pp. 77-98 (1979); Domenico Garozzo, Mario Giuffrida, and Giorgio Montaudo, Macromolecules, vol. 19, pp. 1643-1649 (1986); and, K. Jamshidi, S. H. Hyon and Y. Ikada, Polymer, vol. 29, pp. 2229-2234 (1988).
It is known that lactide polymers exhibit an equilibrium relationship with lactide as represented by the reaction below: ##STR1##
No consensus has been reached as to what the primary degradation pathways are at elevated processing temperatures. One of the proposed reaction pathways includes the reaction of a hydroxyl end group in a "back-biting" reaction to form lactide. This equilibrium reaction is illustrated above. Other proposed reaction pathways include: reaction of the hydroxyl end group in a "back-biting" reaction to form cyclic oligomers, chain scission through hydrolysis of the ester bonds, an intramolecular beta-elimination reaction producing a new acid end group and an unsaturated carbon-carbon bond, and radical chain decomposition reactions. Regardless of the mechanism or mechanisms involved, the fact that substantial degradation occurs at elevated temperatures, such as those used by melt-processors, creates an obstacle to use of lactide polymers as a replacement for petrochemical-based polymers. It is apparent that degradation of the polymer during melt-processing must be reduced to a commercially acceptable rate while the polymer maintains the qualities of biodegradation or compostability which make it so desirable. It is believed this problem has not been addressed prior to the developments disclosed herein.
As indicated above, polylactides have been produced in the past, but primarily for use in medical devices. These polymers exhibit biodegradability, but also a more stringent requirement of being bioresorbable or biocompatible. As disclosed by M. Vert, Die Inqwandte Makromolekulare Chemie, vol. 166-167, pp. 155-168 (1989), "The use of additives is precluded because they can leach out easily in body fluids and then be recognized as toxic, or, at least, they can be the source of fast aging with loss of the properties which motivated their use. Therefore, it is much more suitable to achieve property adjustment through chemical or physical structure factors, even if aging is still a problem." Thus, work aimed at the bioresorbable or biocompatible market focused on polylactide and blends which did not include any additives.
Other disclosures in the medical area include Nieuwenhuis (European Patent No. 0 314 245), Nieuwenhuis (U.S. Pat. No. 5,053,485), Eitenmuller (U.S. Pat. No. 5,108,399), Shinoda (U.S. Pat. No. 5,041,529), Fouty (Canadian Patent No. 808,731), Fouty (Canadian Patent No. 923,245), Schneider (Canadian Patent No. 863,673), and Nakamura et al., Bio. Materials and Clinical Applications, Vol. 7, p. 759 (1987). As disclosed in these references, in the high value, low volume medical specialty market, polylactide or lactide polymers and copolymers can be given the required physical properties by generating lactide of very high purity by means of such methods as solvent extraction and recrystallization followed by polymerization. The polymer generated from this high purity lactide is a very high molecular weight product which will retain its physical properties even if substantial degradation occurs and the molecular weight drops significantly during processing. Also, the polymer may be precipitated from a solvent in order to remove residual monomer and catalysts. Each of these-treatments add stability to the polymer, but clearly at a high cost which would not be feasible for lactide polymer compositions which are to be used to replace inexpensive petrochemical-based polymers in packaging, paper-coating and other non-medical applications.
Furthermore, it is well-known that an increase in molecular weight generally results in an increase in a polymer's viscosity. A viscosity which is too high, can prevent melt-processing of the polymer due to physical/mechanical limitations of the melt-processing equipment. Melt-processing of higher molecular weight polymers generally requires the use of increased temperatures to sufficiently reduce viscosity so that processing can proceed. However, there is an upper limit to temperatures used during processing. Increased temperatures increase degradation of the lactide polymer, as the previously-cited studies disclose.
Jamshidi et al., Polymer, Vol. 29, pp. 2229-2234 (1988) disclose that the glass transition temperature of a lactide polymer, T.sub.g, plateaus at about 57.degree. C. for polylactide having a number average molecular weight of greater than 10,000. It is also disclosed that the melting point, T.sub.m, levels off at about 184.degree. C. for lactide polymers having a number average molecular weight of about 70,000 or higher. This indicates that at a relatively low molecular weight, at least some physical properties of lactide polymers plateau and remain constant.
Sinclair et al. (PCT Application No. WO 92/04413) disclose the use of residual monomer, lactic acid or lactic acid oligomers to plasticize polylactide polymers, with plasticizer levels of 2-60%. Loomis (U.S. Pat. No. 5,076,983) discloses a process for manufacturing a self-supporting film in which the oligomers of hydroxy acids are used as plasticizing agents. Loomis and Sinclair et al. disclose that the use of a plasticizer such as lactide or lactic acid is beneficial to produce more flexible materials which are considered to be preferable. Sinclair et al., however, disclose that residual monomer can deposit out on rollers during processing. Loomis also recognizes that excessive levels of plasticizer can cause unevenness in films and may separate and stick to and foul drums used for casting such films. Thus, plasticizing as recommended, negatively impacts melt-processability.
Accordingly, a need exists for a lactide polymer composition which is melt-stable under the elevated temperatures common to melt-processing. The needed melt-stable polymer composition must also exhibit sufficient compostability or degradability after its useful life in order to be manufactured into useful polymeric articles which are suitable as cost-competitive replacements for similar articles made of petrochemical-based polymers. Further, the melt-stable polymer must be processable in existing melt-processing equipment, by exhibiting sufficiently low viscosities at melt-processing temperatures while polymer degradation remains below a point of substantial degradation and does not cause excessive fouling of processing equipment. Furthermore, the polymer lactide must retain its molecular weight, viscosity and other physical properties within commercially-acceptable levels through melt-processing. It will be further appreciated that a need also exists for a process for manufacturing such polymer compositions. The present invention addresses these needs as well as other problems associated with existing lactide polymer compositions and manufacturing processes therefor. The present invention also offers further advantages over the prior art, and solves other problems associated therewith.